1. Field of the Invention
The invention relates to control for an automatic transmission and, more particularly, to control for a vehicle, which executes coast down control in an accelerator off state.
2. Description of the Related Art
An automatic transmission establishes a plurality of gears having different gear ratios by selectively engaging a plurality of frictional engagement elements, and is often used in automobiles, or the like. When a vehicle equipped with such an automatic transmission is driving, the automatic transmission undergoes shift control on the basis of a shift map that uses a vehicle speed, an accelerator pedal position, and the like, as parameters.
In the thus configured vehicle, for example, Japanese Patent Application Publication No. 2007-146906 (JP-A-2007-146906) describes shift hydraulic control at the time when a downshift due to an increase in the amount of accelerator pedal operation of a driver (power-on downshift) is performed while an upshift due to a decrease in the amount of accelerator pedal operation of the driver (power-off upshift) is being performed.
A hydraulic control device described in JP-A-2007-146906, when it is determined to perform a second shift or a power-on downshift while a first shift or a power-off upshift is being performed, immediately releases an engage-side element of the first shift and a release-side element of the second shift in accordance with the determination of the second shift, while promptly engaging an engage-side element of the second shift in accordance with the determination of the second shift, thus decreasing a turbine rotational speed by engaging the engage-side element of the second shift to proceed with a gear shift.
Incidentally, in the above described shift map, generally, upshift lines from nth gear to (n+1)th gear are set on a higher vehicle speed side than downshift lines from (n+1)th gear to nth gear. Therefore, in shift control using the shift map, there is no possibility that it is determined to perform a downshift from (n+1)th gear to nth gear immediately after start of a power-off upshift from nth gear to (n+1)th gear.
However, when the vehicle is decelerating in a state where the driver has stopped accelerator pedal operation, some vehicles execute downshift control based on a coast down point (coast down speed) different from a shift map used during normal driving (hereinafter, also referred to as “coast down shift control” or “deceleration downshift control”). The coast down speeds are generally set on a higher vehicle speed side than the speeds of downshift lines in an accelerator off state in the shift map. Therefore, as the driver depresses a brake pedal to decelerate the vehicle immediately after the driver stops accelerator pedal operation, a downshift from (n+1)th gear to nth gear based on coast down control (hereinafter, also referred to as “coast downshift” or “deceleration downshift”) is performed immediately after start of a power-off upshift from nth gear to (n+1)th gear based on the shift map.
In this way, when a power-off upshift in execution is stopped and then a deceleration downshift is performed, hydraulic control may not be normally continued to cause an unnecessary decrease in engine rotational speed (turbine rotational speed) depending on the timing at which a deceleration downshift is executed. This makes the driver feel uncomfortable. However, JP-A-2007-146906 does not describe any information about shift hydraulic control at the time when a deceleration downshift is performed during a power-off upshift.